command: initialize the command mode for every command

command: initialize the command mode for every command

All the commands in OpenOCD have been inspected and have the
command mode initialize, apart for two of them.
This is not critical, because the uninitialized value (0) is
equivalent to the enum COMMAND_EXEC, that is also the correct
value for the two mentioned commands.

To keep the code consistent, initialize the command mode to
COMMAND_EXEC.

Change-Id: Iaf043364cbd1005418d787ed045a3ec653612382
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4861
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>